This invention relates to a high pressure discharge lamp of cesium and mercury metal vapors whereby the correlated color temperature (CCT) and color rendering index (CRI) remain constant under dimming conditions. Even when dimmed by up to 40% of its rated power, the lamp has excellent color rendering (greater than 90) and favorable color temperature (3,000 to 4,000xc2x0 K). In the present invention, we operate the lamp in a particular pulsed mode so the efficacy of the lamp is improved considerably while retaining a constant CCT and a constant CRI. We find that when using a fill comprising cesium in the lamp, even when dimmed, a color rendering index over 93-94 can be obtained in such lamps with efficiencies which are substantially improved when compared to lamps operated with a continuous waveform.
High pressure metal vapor lamps are well known in the industry and have been used for a long time. In particular, high pressure sodium (HPS) vapor lamps have been used for street, roadway and high bay area lighting applications. Similarly, high pressure mercury (HPM) vapor lamps are well known and preceded the HPS lamps again for similar applications, mostly outdoors. Over the years, a variety of different metals have been tried to ameliorate some of the deficiencies of these lamps. However, no new practical dominent single species lamps have been introduced to the market over the last thirty years other than HPM and HPS for outdoor general illumination. The problem with HPM is that the efficiency is very low and it emits a very bluish light where the color and the CRI are not very desirable. On the other hand, the HPS lamp that has come after HPM has resulted in very high efficiencies on the order of 120 lpw, but again the CRI and the color are not all that desirable. As it turns out, the CRI is very poor. Many of the colors (red, orange, etc.) do not really appear in their true hue""s under HPS illumination. That is why the majority of the HPS illumination has been confined to outdoor applications rather than indoor. The CCT of the HPS, as it turns out, is mostly on the order of about 2000-2500xc2x0 K. The problem here is that the lamp is not a white light source. It is a very yellowish source and anything below 2500xc2x0 K is not really considered a white light source. Therefore, the appearance of the light source itself, as well as the colors under it, are not very true renditions and it becomes highly undesirable to illuminate indoor objects with these kinds of light sources.
More recently it has been recognized that the color rendition could be improved to a certain degree by increasing the vapor pressure of the sodium in the lamp. The color temperature has been increased to 2800xc2x0 K by using heat bands resulting in higher vapor pressure and whiter appearance. A more recent innovation has been the introduction of white color HPS light sources. These have the characteristics of CCT=2600-2800xc2x0 K with a high color rendering index on the order of about 85. The efficiency, however, of these light sources is considerably lower, down to about 35-45 lpw depending on the power level of the source. Nevertheless, these light sources have found some application in the merchandise and indoor illumination, especially for retail window illumination, nation where red colors need to be accentuated. As it turns out, the red color rendition of the white HPS is very good and some retailers find it very attractive to illuminate red tones with this particular light source even though the efficacy is very low.
The white light has been achieved primarily by either pulsing the light source or increasing the vapor pressure considerably or a combination of both. Since many of these sources are used for specialized indoor applications, they tend to be low wattage, 100 or 200 watt level. The pulsing can be accomplished either by a high frequency or low frequency operation. Although the approach that has been on the market for quite a while, it is usually a hybrid where a magnetic ballast is utilized to run the light source at very high pressure and therefore, the system tends to be somewhat lower cost and one does not have to use an electronic ballast.
However, the whole system tends to be quite bulky and end users sometimes tend to be apprehensive about the size of the ballast. The other important characteristic of the white HPS source is that when it is dimmed, the color tends to deteriorate and it is not white anymore. It is yellowish. Therefore one cannot maintain either the CCT or the CRI of such light sources under dimming conditions. That certainly is a disadvantage in applications where one would like to introduce a certain degree of mood control or energy savings by dimming the light source.
The main objective of the present invention is to provide a dimmable light source with higher efficacy that can be substituted for a white HPS light source with comparable efficacy and having much higher efficiencies than an incandescent tungsten halogen light source.
It is also the objective of the present invention to provide a lamp with a very high CRI, close to 95 at various stages of dimming.
It is also the objective of the present invention is to provide a lamp with color temperatures on the order of 3000-4000xc2x0 K.
Another objective of the present invention is to provide a lamp that can be dimmed to about 40-50% in power without substantially altering the CRI or the CCT.
Yet another objective of the present invention is to provide a dimmable light source that can be manufactured using existing technologies.
A further objective of the present invention is to provide a family of light sources that can be manufactured at higher power levels ranging from about 50 to 1000 watt depending on the requirements of the application.
Another objective of the present invention is to provide a light source that can have a much longer life compared to an incandescent tungsten halogen light source with close to twice the efficacy, at least three to four times the life and yet similar color and CRI with the added advantage of being dimmable without losing the whiteness or the appearance.
We have discovered that the radiation in a high pressure cesium lamp is primarily obtained from a highly ionized cesium plasma as explained herein. We have realized that we could increase the degree of ionization of cesium by a variety of techniques. As it turns out, the cesium has to be highly ionized and excited such that one obtains very broad band radiation. We found that most of the radiation is due to recombination emission and Brehmstrahlung radiation. The reason cesium works so well in this kind of discharge is the ionization potential is very low (3.1 eV) and therefore it is very easy to ionize.
Cesium""s relatively high vapor pressure makes it convenient to operate at reasonable temperatures with a polycrystalline alumina envelope. Also, as it turns out, the cesium resonance emission is near 891 nm and once it is broadened and self-reversed, one branch moves toward the visible and ionic emission is superimposed on it so as to give a spectrum which is very comparable to a tungsten halogen light source and results in high CRI and color temperatures of 3000-4000xc2x0 K depending on the ionization conditions. We have discovered that there are a number of characteristics of the pulsing approach which are unique to these light sources and have been found to be highly beneficial to increase the efficacy but also simultaneously to maintain constant CRI and CCT down to very low dimming levels. Furthermore, we have also found that under certain pulsing conditions we could actually change the color temperature from 3000xc2x0 K to 4000xc2x0 K depending on the application. Therefore a degree of control is provided where we can have a desired CRI and CCT that can be maintained under dimming conditions with an efficacy which is relatively high.
We have found that the light source of the present invention can be operated at a low frequency square wave and one or more current pulses can be superimposed in such a manner that the gradient of the leading edge is very short yielding a high electric field which leads to a high degree of ionization. If the simmer current (that is the arc sustaining current between pulses) is substantially smaller (in the order of 1 to 5%) compared to the peak currents imposed upon the arc tube, we find that the degree of ionization and the excitation of cesium ions is quite high and the efficiency can be improved by as much as 65-110% compared to continuous wave operation. This results in a more attractive light source and still retain the constant CCT and CRI under dimming. Superimposing a single or multiple pulse on a square wave at frequencies anywhere from 125 to 800 Hz has been tried successfully with pulse widths from 50 to 200 xcexcs. The results are shown hereinafter in graphs and comments on the characteristics. These results demonstrated a superior approach although it may require a somewhat more expensive ballast resulting in an overall high performance, higher cost system.